The invention relates to an MRI apparatus for forming an image of an object to be examined in a measuring volume of the apparatus, which apparatus includes
a field magnet for generating a uniform field in the measuring volume of the apparatus,
an RF transmission aerial for transmitting RF radiation of a given wavelength to the object to be examined, and
electrical connection equipment for maintaining a connection with the object to be examined, said connection equipment being coupled to a region outside the measuring volume by means of a conductor provided with frequency-dependent elements.
An apparatus of this kind is known from U.S. Pat. No. 5,602,478. As is customary for MRI equipment, the object to be examined according to the cited patent is formed by a patient; a part of the body of such a patient is to be imaged. As is generally known, the generation of the electromagnetic fields causes an unpleasant noise in MRI equipment. In order to suppress such noise, the known MRI apparatus is provided with electrical connection equipment for maintaining a connection with the patient, the connection equipment in said known apparatus being formed by a headset whereby anti-noise is produced. The headset is coupled to a signal generator which is situated in the region outside the measuring volume. Said coupling is realized via an electrical conductor which consists of two cores and a conductive envelope. In order to prevent undesirable influencing of the imaging by means of the MRI apparatus, the conductor is provided with frequency-dependent elements in the form of a filter circuit which includes a number of filters tuned to a number of frequencies of the RF radiation. Each of these filters is formed as a series connection of an inductance and a capacitor and is arranged between a ground point and a point of the associated conductor which is situated at such a distance from the headset that this distance corresponds to xc2xc wavelength of the RF radiation whereto the relevant filter is tuned. This means that the interference-suppressing effect of said filters is effective for a few, predetermined frequencies only.
The headset used in the known apparatus is a passive element, i.e. the headset does not receive electrical power from outside the measuring volume. However, there is an increasing need for electrical connection equipment for maintaining a connection with the patient to be examined which is capable of receiving electrical power supply energy, such as sensors for monitoring the physical condition of the patient or a TV camera for observing the patient, and the associated lighting.
It is an object of the invention to provide an MRI apparatus of the kind set forth wherein electrical connection equipment with active elements can be used without causing a disturbance of the image for a wide range of radio frequencies used in the apparatus. To this end, the MRI apparatus according to the invention is characterized in that the conductor which couples the connection equipment to the region outside the measuring volume is constructed as a low-frequency conductor for the supply signals of low frequency, relative to the frequency of the RF radiation, to and from the connection equipment,
said low-frequency conductor including segments (72-i) which are separated from one another and each of which is shorter than xc2xc of the wavelength of said RF radiation,
the separation between the segments being realized by frequency-dependent separating elements which constitute a conductor for said low frequencies and an isolator for RF alternating current.
Taking these steps allows for the use of active connection equipment which is fed via a conductive connection, no disturbing effect being exerted by the low-frequency conductors for as long as the segments are significantly shorter than the wavelength of the RF radiation used in the MRI apparatus.
Each of the separated segments in an advantageous embodiment of the invention is shorter than {fraction (1/20)} of the wavelength of said RF radiation. It has been found in practice that such a choice of the length of the segments results in an effect on the MR image which is negligibly small for all practical purposes.
The separating elements in a further embodiment of the invention are formed by inductances which do not contain a ferromagnetic material.
As a result of this step, field concentration in the inductance elements cannot exert an adverse effect on the homogeneity and/or the strength of the homogeneous field generated by the MRI apparatus.
The inductances in another embodiment of the invention are formed in that the input core and the output core in the low-frequency conductor are wound so as to form bifilar wound coils.
As a result of this step, the input core and the output core in the low-frequency conductor will not generate a field outside the inductance elements which could have an adverse effect on the homogeneity and/or the strength of the homogeneous field generated by the MRI apparatus.
The segments in another embodiment of the invention are formed by mutually twisted cores.
As a result of this step, the input and output cores in the low-frequency conductor again will not generate a field which would have an adverse effect on the homogeneity and/or the strength of the homogeneous field generated by the MRI apparatus.